This invention relates to tracking control apparatus and, more particularly, to such apparatus for controlling the scanning trace of at least one transducer as that transducer scans successive parallel tracks on a record medium to reproduce information signals from such parallel tracks without deterioration or interference of such reproduced signals.
In one type of signal recording and/or reproducing apparatus, a rotary head assemby is provided with one or more heads which scan successive parallel tracks on a record medium so as to record and/or reproduce signal information in such tracks. The record medium generally used in such apparatus is magnetic tape, magnetic sheets, magnetic discs or other types of known record media. In general, while the one or more heads rotate across the record medium, the medium itself also is moved. One particular use of this type of apparatus is to record video signal information on, for example, magnetic tape wherein parallel slant tracks are recorded in skewed relation to the longitudinal axis of the tape. Rotary head recording and/or reproducing apparatus also can be used to record other types of information, such as analog signals or, if desired, digital signals. In the interest of simplification, the following explanation and description will refer to video signal recorders and, particularly, video tape recorders (VTR). However, as will be understood, this explanation and description also is applicable to other analog or digital recording/reproducing apparatus which may use magnetic tape or which may use different record media.
A typical VTR includes one, and preferably two, record and/or playback heads mounted on a suitable rotary assembly so as to rotatably scan a magnetic tape which is helically wrapped about at least a portion of guide drum. During recording, a tracking servo system synchronizes the movement of the tape to the rotary movement of the heads, and control signals which can be used for this type of synchronism during a reproducing operation are recorded on the tape. During reproduction, the same or a similar servo control system is used to synchronize the movement of the tape to the rotation of the heads in accordance with the control signals which had been recorded. Consequently, an accurate video picture can be displayed in response to the reproduced video signal. This accuracy is, in large part, due to the fact that servo control system tends to control the relative movement between the rotary heads and tape such that each head scans substantially the same track during a reproducing operation as was scanned during the recording operation.
However, even with this servo control system, there are instances when the path traversed by the heads during reproduction does not coincide satisfactorily with the previously recorded record track. This problem of a deivation or error in the scanning trce of the reproducing head is present particularly when the video signals are recorded by one VTR and are reproduced by a different VTR. If the orbits of the magnetic heads in these different VTR's are not equal to each other, the servo control system may not be effective to bring the heads into precise coincidence with the recorded tracks during reproduction. Other factors also may be present which prevent successful operation of the servo control system.
The aforementioned tracking errors, or scanning deviation, may not be particularly noticeable or objectionable for a VTR wherein guard bands are used to separate adjacent parallel tracks, or when the magnetic tape is transported at a sufficiently high speed. However, if the guard bands between adjacent tracks are reduced, or eliminated, and if the speed at which the magnetic tape is transported is reduced, the effect of tracking errors during a signal reproducing operation may result in deterioration or interference of the reproduced video picture which is not acceptable. It is desirable to minimize or eliminate the guard band, and thus increase the recording density (tracks per inch). and it also is desirable to reduce the transport speed of the magnetic tape so as to increase the amount of video signal information which can be recorded on a magnetic tape of given length. By thus increasing the recording capacity (usually expressed as a measure of time) of the VTR, there need not be a concommitent increase in the supply or length of recording tape. However, for these "long-playing" VTR's, the effect of tracking errors is to reduce the signal-to-noise ratio and, moreover, such errors otherwise deleteriously affect the reproducing characteristics of the VTR.
It is thought that tracking errors of the type wherein a scanning head deviates from the prerecorded track may be corrected by sensing such deviation and by adjusting the position of the head relative to the scanned track so as to minimize such error. If the head is mounted on an adjustable support assembly, and if tracking errors, or deviations, are detected by sensing a corss-talk signal picked up by the head from an adjacent track, then the cross-talk signal can be used to derive a control signal which, in turn, drives the adjustable head support assembly in a direction to bring the head into proper coincidence with the record track, and thus reduce the level of the cross-talk signal. However, with this type of control technique, the head is brought into its correct scanning trace only after a portion of the track has been scanned thereby. This means that a tracking error will exist while the initial portion of the track is scanned, thereby resulting in an undesirable effect upon the first few lines of the reproduced video picture. Since the control signal which is used to properly position the head as it scans the record track is removed or terminated after the head completes its trace, the head returns to its non-controlled position whereat it remains until it is brought into contact with the record medium so as to scan the next track. Hence, there is a need to substantially eliminate the delay in bringing the head into proper position during its trace of a record track. Stated otherwise, there is a need to minimize the time required to displace the head from its non-controlled position to its proper position while scanning the record medium.